Characterization of transformer coupled oxygen plasmas by trace rare gases–optical emission spectroscopy and Langmuir probe analysis

Trace rare gases–optical emission spectroscopy (TRG–OES) and Langmuir probe analysis have been used to measure the electron temperature, Te, in a high-density inductively (transformer) coupled (TCP) 10 mTorr oxygen plasma as a function of the 13.56 MHz radio frequency (rf) power. The oxygen atomic densities were estimated by O-atom optical emission (8446 Å), and rare gas actinometry (Ar, 7504 Å). In the H-(inductive)-mode, Te increases from 2.6 to 3.4 eV for the low-energy electrons sampled by the Langmuir probe and from ∼3.5 to 6.0 eV for the high-energy electrons sensed by TRG–OES as the rf power is increased from 120 to 1046 W. In the E-(capacitive)-mode, below 50 W, Te measured by TRG–OES increases with rf power from ∼4 eV at very low power (∼7 W) to ∼6.1 eV at 45 W. Between the highest E-mode power (∼50 W) and lowest H-mode power (∼120 W), the Te measured by TRG–OES drops from 6.1 to 3.5 eV, while Te derived from Langmuir probe measurements drops only slightly from 3.0 to 2.6 eV. In the H-mode, the electron energy distribution function (EEDF) is bi-Maxwellian from ∼120 to 1046 W. In the E-mode, the EEDF changes from nearly Maxwellian (possibly Druyvesteyn) at low rf powers (∼7 W) to bi-Maxwellian at the higher E-mode powers (∼45 W). O2 dissociation is low (∼2%) at the maximum rf power density of 5.7 W cm−2 (1046 W), and this low value is attributed to the high rate of O-atom recombination on the mostly stainless-steel walls. A detailed accounting of the sources of O (8446 Å) emission revealed significant contributions from electron impact excitation from O(1S) and dissociative excitation of O2.